It is desirable to measure the layer thickness on the article being coated while it is present in the deposition bath, because this is the most practical method to obtain specific layer thicknesses reliably.
For numerous applications, the formation of a coating of a given thickness is of great importance. Falling below this layer thickness leads to performance troubles such as, for example, insufficient anti-corrosion protection or inadequate electrical properties and, therefore, to the formation of scrap. The formation of coatings with thicknesses that are greater than specified may not only lead to the formation of scrap, for instance, for mechanical tolerance reasons. In addition, the deposition process is prolonged through the formation of unnecessarily thick coatings and the amount of material used to form the coating is increased, both of which adversely affect the economics of the process. The costs arising from the formation of excessive coating thickness can be considerable such as, for example, in the production of gold platings, which are being used on a large scale as contact material in modern electrical industry. To ensure reliable contact quality, a specific, prescribed layer thickness is required. It is possible to measure easily and precisely by known processes the layer thickness of the gold plating on an object as soon as the latter has been withdrawn from the bath. If the thickness is insufficient, however, the need to return the object to the bath solution and to continue the deposition process leads to a more complicated and less economical process.
In the case of electrolytic deposition processes, using current strength to determine the quantity of metal deposited is known in the prior art. By this method, however, only the total amount of metal deposited is determined. Moreover, the geometric shape of the objects present in the deposition bath and the influence of neighboring objects and electrolysis aids (so-called pirate power consumers) cause a measurement of this kind to be of doubtful value for the determination of the layer thickness actually obtained at a given site in the deposition bath. Also, this unsatisfactory method can not be used in the case of electroless metal deposition for determining the layer thickness before the coated article is withdrawn from the bath solution, although it is very desirable, for reasons of economy as well as of quality, to be able to know the thickness of the layer that has been formed before withdrawing the coated object from the bath solution. Another disadvantage associated with this prior art method is that the surface being coated becomes passive after being outside the bath solution for only a short time, so that it has to be reactivated if it is found that further metallization is necessary, after the coating thickness has been measured by conventional means.
With electroless metal deposition processes such as, for example, metallization by means of autocatalytic bath solutions operated without an external current supply, it is desirable for optimum operation of the deposition process to have a technique for measuring the deposition rate and its changes at any given time during the carrying out of the process. Measurement of the deposition rate while the deposition process is taking place is also advantageous both in the case of electrolytic and electrophoretic coatings.